The present invention relates to a circuit breaker incorporating a thermal overload open-phase tripping device used in a molded case circuit breaker applied to an electric-motor feeding circuit or the like.
The structure of a conventional circuit breaker, in this example, a molded case circuit breaker, which incorporates a bimetal-type thermal overload open-phase tripping device, is shown in FIGS. 6, 7(a) and 7(b). In these figures, reference numeral 1 denotes a breaker case (resin molding), reference numeral 2 denotes a case cover (resin molding) placed on the case 1, reference numerals 3 and 4 denote load-side main circuit terminals and power-supply-side main circuit terminals, reference numeral 5 denotes an opening and closing mechanism section for a main circuit contact shoe, reference numeral 6 denotes an opening and closing operating handle, reference numeral 7 denotes a bimetal-type thermal overload open-phase tripping device incorporated in the case 1, and reference numeral 8 denotes an instantaneous-operation-type solenoid-based overload tripping device. Although not shown, a main circuit breaking section composed of movable and fixed contact shoes and an arc extinguishing chamber is incorporated into a bottom side of the case 1.
Further, the thermal overload open-phase tripping device 7 includes bimetals 9U, 9V and 9W corresponding to different phases of a main circuit, a differential shifter mechanism 10, a compensating bimetal 11 also acting as a lever for linking the differential shifter mechanism 10 and a latch receiver 5a of the opening and closing mechanism section 5, an adjustable dial 11a for the compensating bimetal 11 and the like.
The interior of the case 1 is partitioned into chambers corresponding to the U, V, and W phases by interphase partition walls 1a. The bimetals 9U, 9V and 9W are laterally arranged in the chambers for the respective phases and have lower ends fixed to and supported by the case 1, as shown in FIG. 7(a). Heater wires 9a wound around the corresponding bimetals are connected to the main circuit to cause the bimetals to bend when a current is conducted through the heater wires to generate heat.
On the other hand, the differential shifter mechanism 10 is composed of an assembly of a plate-shaped push shifter 10a and a plate-shaped pull shifter 10b, arranged alongside the bimetals 9U, 9V and 9W, and an interlocking plate 10c extending over the top surfaces of the push shifter 10a and the pull shifter 10b. Each of the L-shaped arm portions extending laterally from the push shifter 10a and a corresponding L-shaped arm portion extending laterally from the pull shifter lob face each other across a top portion of the corresponding bimetal 9U, 9V or 9W, and the corresponding bimetal then acts as an operating end. Further, the interlocking plate 10c has an output end or projecting portion 10c-1 located at a side opposite to a side surface of the compensating bimetal 11.
The interphase partition walls 1a of the case 1 have recesses 1a-1 formed at upper edges thereof, in which the push shifter 10a and the pull shifter 10b are fitted, as shown in FIG. 7(b). The push shifter 10a and the pull shifter 10b extend over the interphase partition walls 1a and lie within the recesses 1a-1 so as to be loosely fitted therein to permit sliding. In an assembled state in which the case cover 2 is installed on the case 1, as shown in FIG. 7(a), the case cover 2 holds the differential shifter mechanism 10 to keep it from slipping from the recesses 1a-1 in the interphase partition walls 1a. 
Since the operation of the thermal overload open-phase tripping device configured as described above is well known, a detailed description thereof is omitted. When an overcurrent flows through the main circuit to bend and displace the bimetals 9U, 9V and 9W in the direction of arrow P in FIG. 7(a), the push shifter 10a of the differential shifter mechanism 10 correspondingly slides in the direction of arrow Q to cause the output end of the interlocking plate 10c to push the compensating bimetal 11, thereby driving the latch receiver 5a (see FIG. 6) of the opening and closing mechanism section 5 to a release position. The opening and closing mechanism section 5 thereby performs a trip operation to open the contact shoes of the main circuit of the breaker, thereby shutting off the current.
The circuit breaker of the above conventional configuration has the problem such that overload open-phase tripping operation characteristics change, resulting in failure to operate as specified. The inventors"" study has demonstrated that this problem originates from the support structure of the differential shifter mechanism. That is, in the differential shifter mechanism 10, the interlocking plate 10c is placed on the top surfaces of the push shifter 10a and the pull shifter 10b so as to extend over these surfaces, as previously described. Thus, in the conventional assembled structure, a gap corresponding to the thickness of the interlocking plate 10c remains between the top surfaces of the push shifter 10a and the pull shifter 10b and an inner wall surface of the case cover 2, as shown in FIG. 7(a).
Consequently, when the push shifter 10a of the differential shifter mechanism 10 is pushed due to the bending displacement of the bimetals 9U, 9V and 9W, it is inclined, using as a support point the abutting point between the output end of the interlocking plate 10c and the compensating bimetal 11, so that its rear end (right end in the figure) is raised. Thus, the rear edge of the raised shifter rubs against the inner surface of the case cover 2 to produce the sliding friction, and the inclined position of the shifter reduces the force for laterally pushing the compensating bimetal 11 via the interlocking plate 10c. As a result, the bending displacement of the bimetals 9 is not accurately transmitted to the compensating bimetal 11 via the differential shifter mechanism 10, varying the tripping operation characteristics of the breaker.
It is thus an object of the present invention to provide a circuit breaker that provides an improved support structure for a differential shifter mechanism so as to accurately transmit the bending displacement of bimetals to a compensating bimetal via a differential shifter mechanism without any loss, in order to stabilize the operational characteristics of a thermal overload open-phase tripping device.
Further objects and advantages of the invention will be apparent from the following description of the invention.
To attain the above object, the present invention provides a circuit breaker including a thermal overload open-phase tripping device. The tripping device is composed of bimetals corresponding to different phases of a main circuit; a differential shifter mechanism composed of a combination of a plate-shaped push shifter and a plate-shaped pull shifter linked to operating ends of the bimetals of the respective phases, and an interlocking plate extending over the top surfaces of both shifters; and a compensating bimetal also acting as a lever to transmit an output from the shifter mechanism to an opening and closing mechanism section to release a latch receiver. The push and pull shifters of the differential shifter mechanism extend over interphase partition walls in a breaker case and are loosely fitted into recesses formed at the upper edges of the interphase partition walls to permit sliding support. A case cover is placed and held over the push and pull shifters of the differential shifter mechanism.
In the invention, shifter position guides are provided on an inner surface side of the case cover opposite to the push and pull shifters to restrain inclination of the shifters moving in accordance with bending displacement of the bimetals to hold the shifters horizontally.
The position guides may be configured as rib-shaped projections projecting from an inner surface of the case cover along a shifter operating direction toward the top surfaces of rear portions of the push and pull shifters.
The position guides may be configured as rib-shaped projections projecting from an inner surface of the case cover along a direction traversing the top surfaces of the push and pull shifters and parallel to the interphase partition walls in the case.
In the above two situations, the tips of the rib-shaped projections and inner surfaces of the recesses in the interphase partition walls in which the push and pull shifters are loosely fitted for support are formed to have circular cross-sections, respectively.
According to the above configuration, in the differential shifter mechanism incorporated in the breaker case, the position guides projecting downward from the inner surface of the case cover are located opposite to the top surfaces of the push and pull shifters fitted into the recesses formed at the upper edges of the interphase partition walls of the case, in such a manner that a small gap is maintained to allow sliding movement of the shifters between the top surfaces and the position guides. If the shifters moving in accordance with the bending of the bimetals begin to incline forward, they abut against the position guides, thereby preventing their inclination. This allows the shifters to slide while maintaining their horizontal positions.
Furthermore, since the tips of the rib-shaped projections and the inner surfaces of the recesses in the interphase partition walls are formed to have circular cross-sections, respectively, the shifters slide smoothly without catching any part, reducing the sliding friction resistance. As a result, the bending displacement of the bimetals is accurately transmitted to the compensating bimetal via the differential shifter mechanism without any loss, thus stabilizing the tripping operation characteristic of the circuit breaker.